Cancer remains a significant health problem throughout the world. Current therapies, which are generally based on a combination of chemotherapy or surgery and radiation, continue to prove inadequate in many patients.
Among cancers, melanoma is well known both for its rapidly increasing incidence and its resistance to virtually all but surgical therapies. Melanoma arises from melanocytes, neural crest derived pigment cells in the skin and eye. During melanoma carcinogenesis, many of the normal markers of the melanocyte lineage become lost. Gene expression patterns in melanoma cells and melanocytes have significant differences that reflect the cancerous nature of melanoma. In general, gene expression is regulated by two types of factors—DNA binding transcription factors and co-regulators which form cell type specific complexes including mediator complex and chromatin remodeling complex that control activity of RNA polymerase two (pol II). Cofactor complexes integrate signals from DNA binding transcription factors as well as from different signaling systems to control RNA synthesis. Cofactor complexes are highly cell- and stimulus-specific, and vary from one physiological stage to another. Cancer cells express transcriptional co-factors with modified structure that is a result of mutations, post-translational modifications, alternative splicing, fusion of different fragments of different proteins to name but a few,
Transcriptional Control of Melanoma and Melanocyte Development
Despite altered gene expression patterns, most, if not all, melanomas retain expression of the basic/helix-loop-helix/leucine-zipper (bHLHzip) transcription factor Microphthalmia-associated transcription factor (MITF) (King et al., 1999) that is characteristic for melanocytes. Published data suggests a role for MITF in the commitment, proliferation, and survival of melanocytes before and/or during neural crest cell migration (Opdecamp et al, 1997). Numerous studies also suggest that MITF, in addition to its role in differentiation pathways such as pigmentation, may have an important role in the proliferation and/or survival of developing melanocytes. The retention of MITF expression in the vast majority of human primary melanomas, including nonpigmented tumors, is consistent with this possibility and has also led to the widespread use of MITF as a diagnostic marker in this malignancy (King et al., 1999; Salti et al. 2000; Chang and Folpe, 2001; Miettinen et al., 2001). Wnt signaling pathway and beta-catenin are significant regulators of melanoma cell growth, with MITF as a critical downstream target. Importantly, disruption of the canonical Wnt pathway abrogates growth of melanoma cells, and constitutive overexpression of MITF rescues the growth suppression.
The invention disclosed herein arises from a search for MITF target genes, which influence cell cycle progression, to examine the possibility that MITF contributes to maintenance of the cell cycle machinery while perhaps not directly participating in the mitogenic response. Cell cycle targets of Wnt signaling such as c-Myc, Cyclin D1 (He et al., 1998; Tetsu and McCormick, 1999; Shtutman et al., 1999), and others may more directly mediate beta-catenin's mitogenic effects. In addition, it has been shown that MITF serves as an upstream regulator of a variety of proliferation related genes such as CDK2: p21 (Cip1): INK4A. MITF interacts with several transcription factors (TEs) including Rb, TFEB, ITF2, PIAS3 and STAT3, to regulate a network of downstream genes that are related to different aspects of melanocyte and melanoma development.
In addition to the MITF pathway, several other signaling pathways have been reported to be associated with melanoma cells, including NOTCH, interferon, nuclear hormone receptor and immune modulatory pathways. Some differentially expressed genes reside on chromosomal regions displaying common loss or gain in melanomas or are known to be regulated by CpG promoter methylation. Several data also indicate that transcription cofactors are differentially expressed in melanomas compared to melanocytes, Goldberg et al. (2003) reported that tumor suppressor genes TXNIP and KISS1, which are down-regulated in metastatic melanomas, are controlled by transcriptional factor DRIP130CRSP3. DRIP130/CRSP3 is located in chromosome 6 in the region that is frequently deleted in melanomas.
Transcriptional Control
Precise temporal and spatial regulation of the transcription of protein-encoding genes by RNA polymerase II (pol II) is vital to the execution of complex gene expression programs in response to growth, developmental and homeostatic signals. The molecular circuitry that enables coordinated gene expression is largely based on DNA-binding transcription factors (TFs) that bring regulatory information to the target genes. As a rule, DNA binding TFs do not interact directly with pol II and other basal transcriptional complex components. Group of factors called co-regulators including co-activators, co-repressors and a mediator complex have emerged as central players in the process of transcription. These co-regulators mediate DNA binding TFs and pol II complex to control transcriptional activity of specific genes.
Although it has been realized that co-regulators are universally required for the expression of almost all genes, the full implications of a requirement for a multi-subunit co-regulator complex are not yet readily apparent. By inserting itself between the DNA binding TFs and the basal transcriptional machinery, the mediator complex probably affords additional opportunities to control the diverse regulatory inputs received both from the DNA-binding factors and, most likely, from other signals and to present an appropriately calibrated output to the pol II machinery. In its capacity as a processor of diverse signals in the form of activators and repressors that impinge on it, and its location at the interface of pol II and general transcription factors (GTFs), the mediator represents a final check-point before pol II transcription actually commences. The central role of co-regulator complexes in transcriptional control makes them an attractive drug target. Interference at this point of transcription machinery could enable researchers and clinicians to control or correct expression of a large number of genes. Transcriptional complex that contains 70-80 subunits has a different composition in different cell types and on different promoters. This cell specific variability of transcriptional complex assures specificity of potential treatments that target transcriptional machinery.
There remains a need for molecules useful in the treatment of cancer. The invention disclosed herein meets this need by providing isoforms of transcription factors and molecules that specifically target the transcription complexes found in cancer.